Speaking to new baddie Krasko (Joshua Bowman) during this week’s episode, the Doctor (Jodie Whittaker) quickly surmises that he’s an intergalactic criminal based on the distinctive tattoo on his wrist, which belongs to a formidable prison.

“First things first – tell me about Stormcage,” the Doctor says to Krasko.

“On your wrist, there’s a Stormcage identifier. The most secure facility this side of the universe.”

And Who fans will instantly recognise Stormcage as the name of the prison that River Song was imprisoned in for much of Matt Smith’s run as the Eleventh Doctor, after she was put away for “killing” the Doctor (actually a Tesseract duplicate) and given twelve thousand consecutive life sentences.

Stormcage made several appearances throughout the series after its first mention, with River often escaping from it to spend time with the Doctor, so it’s a fun little nod for Chibnall and episode writer Malorie Blackman to include.

But technically, it might not be the only River Song Easter Egg in the episode. The Doctor also notes that Krasko uses a vortex manipulator to travel through time, a “cheap and nasty” form of time travel that has appeared in the series since 2005, and is most associated with John Barrowman’s “Time Agent” character Captain Jack Harkness.

However, vortex manipulators were also regularly used by River in her adventures (and escapes from prison) – and given that Krasko says he picked up his through bartering inside Stormcage, it’s not completely out of the question that his particular device is one of River’s cast-offs that she left behind.

Now all we need is for River herself to make an appearance, and add to her tally of Doctors. She’s been onscreen with Ten, Eleven and Twelve and met a few of the others in Big Finish audio dramas – but will she now add Thirteen to the list?